So far we have presented a lot of really cool papers, but today I want to talk about a paper that looks incredibly cool in a whole bunch of different ways in the abstract, introduction, and discussion but for whom much of that awesomeness falls apart under closer inspection of the results, methods, and context. In it, the authors report their results having flown in NASA’s venerable old DC-8 across the US and down the west coast as well as through a couple of hurricanes with a filter designed to capture bacteria sized particles hanging out the side. They then took the filter and analyzed it with fluorescent dyes and microscopes as well as genomically to see what was there. In their paper they appear to arrive at five different major conclusions: that viable bacterial cells represented on average around 20% of the total particles in the 0.25- to 1-μm diameter range; that 60 to 100% of the 1.5 × 105 cells m−3 they saw were viable; that bacteria are at least two orders of magnitude more abundant than fungal cells in the troposphere; and that fecal coliforms represent a significant amount of the microbiota of hurricanes after landfall. What has made the most splash, though, is their speculation that because some of the taxa they determined were present by small subunit rRNA sequencing (See previously) had been shown to metabolize oxalic acid, a major chemical component of clouds, it was plausible that there was active bacterial metabolism happening in the clouds they analyzed. Unfortunately, despite the journal it is published in and glowing praise from excellent blogs like Not Exactly Rocket Science, Climate Central, Wired, Metafilter, and The Scientist, the speculation is pretty foolish and each of these conclusions is either inherently false, actively misleading, or very difficult to support with their data.
Here is the paper:
Microbiome of the upper troposphere: Species composition and prevalence, effects of tropical storms, and atmospheric implications
N DeLeon-Rodriguez, TL Lathem, LM Rodriguez-R, et al. Published 2013 in PNAS. doi: 10.1073/pnas.1212089110
The composition and prevalence of microorganisms in the middle-to-upper troposphere (8–15 km altitude) and their role in aerosol-cloud-precipitation interactions represent important, unresolved questions for biological and atmospheric science. In particular, airborne microorganisms above the oceans remain essentially uncharacterized, as most work to date is restricted to samples taken near the Earth’s surface. Here we report on the microbiome of low- and high-altitude air masses sampled onboard the National Aeronautics and Space Administration DC-8 platform during the 2010 Genesis and Rapid Intensification Processes campaign in the Caribbean Sea. The samples were collected in cloudy and cloud-free air masses before, during, and after two major tropical hurricanes, Earl and Karl. Quantitative PCR and microscopy revealed that viable bacterial cells represented on average around 20% of the total particles in the 0.25- to 1-μm diameter range and were at least an order of magnitude more abundant than fungal cells, suggesting that bacteria represent an important and underestimated fraction of micrometer-sized atmospheric aerosols. The samples from the two hurricanes were characterized by significantly different bacterial communities, revealing that hurricanes aerosolize a large amount of new cells. Nonetheless, 17 bacterial taxa, including taxa that are known to use C1–C4 carbon compounds present in the atmosphere, were found in all samples, indicating that these organisms possess traits that allow survival in the troposphere. The findings presented here suggest that the microbiome is a dynamic and underappreciated aspect of the upper troposphere with potentially important impacts on the hydrological cycle, clouds, and climate.